The present invention relates to high temperature nickel-iron-chromium-base alloys, having a specific hardening precipitation.
The present alloys are derived from an alloy described in U.S. Pat. No. 3,046,108 which includes Cr: 15 to 25 percent; Fe: up to 30 percent; Ni: 30 to 58 percent; Mo: 2 to 7 percent; C: up to 0.2 percent; Si: up to 0.5 percent; (Nb + Ta): 3 to 8 percent, (Ti + Al): 0.4 to 2.5 percent. These alloys contain a considerable quantity of iron which maintains high-level mechanical resistance and resistance to oxidation while limiting cost by limiting the nickel content. Chromium is essential to assure proper resistance against oxidation. A low silicon content is indispensable to obtain a good welding point. Present in low quantity, carbon takes a part in the material's resistance to distortion because, during thermal treatments, it results in the precipitation of localized carbides at the joints of the grains. During thermal treatment, the additional elements, namely, niobium, tantalium, titanium and aluminium induce formations of particles of intermetallic phases which cause a hardening called "structural hardening."
To obtain structural hardening, the alloy is first subjected to a thermal treatment which brings the additional elements into solution; next, it is subjected to hardening, to bring the solution to a metastable state; precipitation not yet having occurred. This metastable solution state has a tendency to regain its equilibrium by rejecting a part of the additional elements as precipitates. Titanium and aluminium cause the formation of a cubic intermetallic phase with centered faces (CFC) of the L1.sub.2 type which has a chemical composition of the A.sub.3 B type, with A mostly representing nickel and B, mostly titamium and aluminium. This phase is designated by .gamma.'.
Niobium and tantalium cause the formation of an intermetallic phase of a centered tetragonal structure type D022, which also has an A.sub.3 B type composition, with A representing nickel and B niobium and/or tantalium. This phase is designated by .gamma.".
The precipitation of this known alloy has been studied by PAULONIS, OBLAK AND DUVAL in an article entitled "Precipitation in Nickel-Base Alloy F18" published in "Transactions of the ASM" in 1969. These authors were the first to show that type .gamma.' and type .gamma." phases exist simultaneously in the alloy described in the previously mentioned patent. They have also shown that the .gamma." precipitates appear as thin, small plates while the .gamma.' precipitates are generally spherical or cubical in shape. The .gamma.' and .gamma." precipitates increase independently. They are sometimes separated in the matrix. The .gamma.' particles can also be joined to some .gamma." particles with the .gamma.' phase, playing a minor part in the hardening.
These authors have shown that the .gamma." is susceptible of a rapid coalescence at temperatures above 650.degree.C which leads to a decrease in resistance.
The hardness of the alloy D, is known as a function of aging time T, at 750.degree.C also shows that hardness D decreases rapidly during a prolonged exposure to this temperature. Modifications of the structure at temperatures above 650.degree.C limit the use of the alloy to parts which will not be thermally exposed. Uses at temperatures above 650.degree. can lead to premature breaks in service.